As a heat-sealable polyolefin composite film conventionally used for packaging and the like, a composite film formed by laminating a polyolefin resin having a low melting point on a crystalline polypropylene surface layer is known.
For example, the following methods have been specifically proposed. A polypropylene resin composition containing 1 to 50 wt % of a propylene polymer having a melting point of not lower than 150° C., 10 to 90 wt % of a propylene copolymer having a melting point of 100° C. or higher and lower than 150° C. and 1 to 80 wt % of a propylene or 1-butene copolymer having a melting point of 40° C. or higher and lower than 100° C. is laminated as a heat-fusible layer on one side of a biaxially-stretched polypropylene film, to thereby form a stretched film (Patent Literature 1: WO2007/015415). In another example, a coating layer composed of a propylene polymer composition containing a propylene/α-olefin random copolymer having a melting point of 120 to 140° C., a propylene/a-olefin random copolymer having a melting point of 80° C. or higher and lower than 120° C., and a butene-1/α-olefin random copolymer having a melting point of 40 to 100° C. is laminated on both sides of a biaxially-stretched film mainly composed of a propylene polymer, to thereby form a over-wrap packaging film (Patent Literature 2: JP-A-2003-170554). As described above, a composition containing two or more kinds of propylene copolymers differing in melting point is used as a surface layer (heat-sealable layer).
In addition to the two-layer film, a three-layer film has also been proposed. For example, an intermediate layer is provided between a base layer which is a biaxially-stretched polypropylene film and a surface layer composed of a composition containing two or more kinds of propylene copolymers differing in melting point, the intermediate layer being composed of a composition of a propylene homopolymer and a propylene/α-olefin copolymer, to thereby provide a heat-sealable laminated polypropylene resin film (Patent Literature 3: JP-A-2005-104151, Patent Literature 4: JP-A-2005-305998). In another example, a composition of propylene homopolymer and a propylene/α-olefin copolymer is used as a base layer; a composition of a propylene homopolymer and a propylene/α-olefin copolymer is used as an intermediate layer; and a propylene/α-olefin copolymer having a melting point of not higher than 150° C. or its composition is used as a surface layer (heat-fusible layer) (Patent Literature 5: JP-A-2006-305884, Patent Literature 6: JP-A-2006-327183).
However, the heat-sealable laminated polypropylene resin film using an intermediate layer specifically proposed in Patent Literature 3 or Patent Literature 4 is inferior in heat sealability.